The scientist who proposed the theory of evolution is Charles Darwin. He introduced the concept in his groundbreaking work "On the Origin of Species," published in 1859. Darwin's theory of evolution by natural selection suggests that species evolve over time through a process wherein organisms with favorable traits are more likely to survive and reproduce. This fundamental idea revolutionized the understanding of biological diversity and laid the foundation for modern evolutionary biology. Darwin's extensive observations during his voyage on the HMS Beagle contributed significantly to the development of this theory.